In a vehicle on which an engine (internal combustion engine) is mounted, an automatic transmission that automatically sets an optimal gear ratio between the engine and drive wheels is known as a transmission that appropriately transmits a torque and a rotational speed generated by the engine to the drive wheels according to a travel state of the vehicle. Automatic transmissions mounted on a vehicle include, for example, a planetary gear transmission that sets the gear ratio using a planetary gear device and frictionally engaging elements such as a clutch and a brake, and a belt type continuously variable transmission (CVT) that continuously adjust the gear ratio.
The belt type continuously variable transmission (hereinafter also simply referred to as the “continuously variable transmission”) includes: a primary pulley (input side pulley) to which a drive force of the engine is transmitted; a secondary pulley (output side pulley) that is connected to the drive wheels (output shaft); and a belt that is wound on the primary pulley and the secondary pulley. By decreasing a width of a pulley groove of one pulley while increasing the width of the pulley groove of the other pulley, the winding radius (effective diameter) of the belt to each of the pulleys is continuously changed to adjust the gear ratio. Specifically, each of the primary pulley and the secondary pulley includes a fixed sheave and a movable sheave, and the movable sheave is moved forwards and backwards in an axial direction using a hydraulic pressure actuator provided at a rear face side thereof so that the gear ratio is continuously adjusted within the range from the minimum gear ratio γmin (highest speed gear ratio: maximum High) to the maximum gear ratio γmax (lowest speed gear ratio: maximum Low).
In such a continuously variable transmission, a hydraulic pressure supplied from a mechanical oil pump that is driven by the engine is adjusted so that the adjusted hydraulic pressure is supplied to the respective hydraulic pressure actuators of the primary pulley and the secondary pulley, thereby performing speed change control.
For example, when the gear ratio is decreased, the hydraulic pressure (hydraulic pressure for speed change control) in the hydraulic pressure actuator (oil chamber) of the primary pulley is increased so that the groove width of the primary pulley is decreased, thus the winding radius of the belt in the primary pulley is increased. Simultaneously, the hydraulic pressure (hydraulic pressure for clamping force control) in the hydraulic pressure actuator (oil chamber) of the secondary pulley is decreased so that the groove width of the secondary pulley is increased, thus the winding radius of the belt in the secondary pulley is decreased while the belt clamping force is adjusted. In this way, it is possible to change the gear ratio while suppressing a belt slip. Also, when the gear ratio is increased, the hydraulic pressure for speed change control is decreased so that the groove width of the primary pulley is increased, thus the winding radius of the belt in the primary pulley is decreased, while the hydraulic pressure for clamping force control is increased to decrease the groove width of the secondary pulley, thereby increasing the winding radius of the belt in the secondary pulley.
Among the continuously variable transmissions, there is a type that is capable of mechanically locking the primary pulley at the time of the maximum gear ratio γmax. For example, at the time of the maximum gear ratio γmax, the movable sheave of the primary pulley makes contact with a wall on the side of the case, which prevents further movement of the movable sheave (in the direction in which the movable sheave is opened), thereby the primary pulley (movable sheave) is mechanically locked (see, for example, Patent Document 1). With such a configuration of the continuously variable transmission, since the movable sheave of the primary pulley makes contact with the wall on the side of the case at the time of the maximum gear ratio γmax (i.e. the movement of the movable sheave in the direction in which it is opened is prevented), it is possible to maintain (lock) the groove width of the primary pulley by a reaction force (belt tension) of the secondary pulley. Thus, when the gear ratio of the continuously variable transmission is the maximum gear ratio γmax, it is possible to decrease the hydraulic pressure by such a degree that the hydraulic pressure of the primary pulley is not needed to be secured (that the primary pulley can be mechanically locked). That is, at the time of the maximum gear ratio γmax, it is sufficient to secure the hydraulic pressure of only the secondary pulley.
In the art related to control of the continuously variable transmission, the art to secure a restart performance from a stop state of a vehicle is provided, in which the gear ratio of the continuously variable transmission is returned to the maximum decelerating side immediately before the vehicle stops (when the vehicle speed is extremely low), and furthermore an engine torque is temporary increased during turning-to-low (see, for example, Patent Document 2).